The invention relates to a reinforcing structure, comprising metallic elements, and the use of such reinforcing structure to reinforce stiff composite articles. The invention further relates to stiff composite articles and a method to provide stiff composite articles.
Reinforcing structure are known to reinforce polymer articles. Often, glass fibers or C-fibers are used to reinforce polymer matrices, providing together a reinforced, eventually shaped article. During forming processes, the bending properties of the polymer matrix are influenced negatively, since the reinforcing structures are difficult to elongate.
Often, metallic filaments or fibers are integrated into composite materials in order to obtain a material that is EMI shielded. As a result of electromagnetic requirements, metallic filaments or fibers with fine diameters of 100 xcexcm or less are used. An example of this can be found in U.S. Pat. No. 5,089,326. It is known in the art that the addition of fine metallic fibers or filaments, usually less than 100 xcexcm, does not increase the strength of the composite material.
Metal wires or metal wire textile fabrics are also known as reinforcing structures, e.g. from FR1290278, EP234463A1, EP546962A1 or EP392904A1. To improve the deformability of the composite material, metal wires are preferably provided as knitted fabrics.
The present invention relates to a reinforcing structure comprising metallic elements. The reinforcing structure can be used to manufacture stiff composite articles, which further comprises a polymer matrix.
It is an object of the present invention to provide a reinforcing structure which comprises metallic elements, all metallic elements laying essentially parallel to each other, which has improved bending properties compared to the known wires or wire meshes, and which provide an alternative to the knitted wire structures.
A reinforcing structure as subject of the invention comprises metallic elements, which are all essentially parallel to each other in the reinforcing structure, wherein each of the metallic elements are structurally deformed. By xe2x80x9cessentially parallel, xe2x80x9d it is meant that due to the nature of the metallic elements, the distance between adjacent metallic elements may vary only slightly.
In the scope of the present invention, metallic elements are to be understood as metallic wires, e.g. drawn metallic wires, a bundle of metallic wires, metallic strands or cords.
As subject of the invention, the bending properties of reinforcing structure are improved by integrating metallic elements in the reinforcing structure, which have an elongation at rupture of more than 3% compared to initial length. This elongation can even be more than 5% or more than 7%, e.g. more than 10% and most preferentially more than 15% at rupture.
According to the invention, this elongation is obtained by the use of a metallic element having structural deformations. Such structural deformation may have an irregular shape, e.g. undulated with a wavelength and/or amplitude, which varies over the length to the metallic element. Preferably however, the metallic element are undulated or spirally shaped in a regular way, with constant parameters over the whole length of the metallic element.
A structural deformation can be e.g. an undulation. The metallic element, being a wire, strand or cord, is given a form, characterized by a wavelength and an amplitude. This undulated metallic element is essentially comprehended in one plane, parallel to the axis of the metallic element. Another structural deformation can be that e.g. a metallic element has obtained a spiral shape, characterized by a diameter of the spiral and the length of the metallic element, necessary to make a 360xc2x0 revolution in the spiral shape.
In case the metallic element is undulated, this undulation being the structural deformation, the removal of the structural deformation of the metallic element will lead to an elastic elongation, a plastic elongation, or a combination of both of the metallic element during the forming process, mainly depending on the wavelength of the undulation.
In case a small wavelength is used, e.g. less than 3 mm, a higher force will be necessary to elongate the metallic element, so removing the structural deformation, next to the elastic and plastic elongation of the metallic element under load as if there was no structural deformation. The removing of this structural deformation leads to a mainly plastic elongation of the metallic element. When the metallic element is released, the metallic element will not come back to its original undulated form, but will stay elongated to certain extend.
In case a longer wavelength is applied, e.g. more than 3 mm or even more than 4.5 mm, a lower force is sufficient to stretch the metallic element, so removing to some extend the structural deformation. This leads to a mainly elastic elongation of the metallic element, which is additional to the elastic elongation of the metallic element under low force as if there was no structural deformation. When the metallic element is released, the metallic element will come back to approximately its original undulated form. When higher forces are applied, a plastic elongation of the metallic element itself is found, next to the removal of the remaining structural deformation.
The elongation of the metallic elements being undulated with a large wavelength, can be more than 0.3%, e.g. more than 0.4%, or even more than 0.5%, preferably more than 1% or more, applying a force of 10% of the force at rupture of the metallic element.
In case the metallic element has a spiral shape structural deformation, an identical behavior is obtainable. Depending on the length to make a 360xc2x0 rotation and the diameter of the spiral, the removal of the structural deformation of the metallic element will lead to an elastic elongation, a plastic elongation or a combination of both of the metallic element during the forming process.
One understands that the elongation due to a removing of the structural deformation, can be either elastic, or plastic or a combination of a plastic and an elastic elongation, depending on the wavelength or the length to make a 360xc2x0 revolution applied.
Metallic elements, being part of a reinforcing structure as subject of the invention, may have more than one structural deformation, being e.g. undulations or spiral shape structural deformations, superposed one to another.
When a metallic element having two structural deformations, being two undulations of which one has a large wavelength and the second having a short wavelength, is subjected to an elongation force F, a stress-strain curve comprising different successive zones, limited by forces F1, F2, F3 and F4 (F1 less than F2 less than F3 less than F4), is obtained.
For a force F applied less than F1, an elastic elongation is obtained. When a force F larger than F1, but less than F2 is applied, an additional, essentially plastic elongation is added. These elongations can be explained by the mainly removing of the structural deformation with large wavelength.
When the force F is increased to a level between F2 and F3, the additional elongation again is mainly elastic. When F is increased to a level between F3 and F4, the elongation in this zone becomes mainly plastic, until rupture occurs at F4. These two later elongations are mainly explained by the removal of the structural deformation with short wavelength, and the plastic deformation of the metallic element without structural deformation until rupture.
The forces F1, F2 and F3, where the elongation change from elastic to plastic or vice versa, can be adjusted by the wavelength and amplitude of the undulations together with other parameters of the metallic element, such as diameter of wires, alloy and drawing history and eventually strand and cord construction.
In case more than 2 structural deformations are superposed to each other, a similar behavior of the metallic element under load can be obtained.
In case the structural deformations are undulations, the metallic element may either be undulated in one plane parallel to the axis of the metallic element, or the undulations may be provided in different planes, all parallel to the axis of the metallic element.
The same characteristics for metallic elements are found in case one or more structural deformations are spiral shaped structural deformations.
The reinforcing structure may comprise one or more layers, hereafter called reinforcing layers. According to the present invention, the metallic elements are present in at least one of the reinforcing layers, all metallic elements of this reinforcing layer are running essentially parallel to each other. In case the reinforcing structure comprises more than one reinforcing layer, of which at least two reinforcing layers comprise metallic elements, the direction of the metallic elements of one layer may be different of the direction of the other layer.
The reinforcing structure is to be understood as all elements, which are used to provide the stiff composite article, next to the polymer matrix. A person skilled in the art understands that also other materials can be used to provide elements of the reinforcing structure, e.g. glass fiber, C-fiber or mats or woven fabrics out of such materials, in addition to the metallic elements.
An object of the invention is to improve the bending properties and behavior of reinforcing structures comprising metallic elements during a forming process of a stiff composite article, comprising this reinforcing structure and a polymer matrix. During this process, the reinforcing structure and the polymer matrix are subjected to heat and/or pressure in order to obtain a stiff composite article. The forming can be done in one step or may require subsequent steps.
It was found that when a reinforcing structure comprising metallic elements, having a structural deformation, is subjected to a forming process, the force needed to form the stiff composite article is partially used to elongate the metallic element. The force applied will elongate the metallic element by partially removing the structural deformations. This is obtainable because of the elongation at rupture of the metallic elements as indicated above.
A reinforcing structure as subject of the invention may be subjected to a forming process, together with the polymer matrix. This process comprises a heating action to soften the polymer matrix, a forming action to form the softened matrix and the reinforcing structure in a desired form, and a cooling action to stiffen the matrix again.
When a metallic element having more than one structural deformation is used as a part of the reinforcing structure of a stiff composite article, benefits are provided to the bending properties during the forming process on the one hand and to the stiff composite article on the other hand. In case the force F, used to provide a stiff composite article comprising the reinforcing structure is approximately F2, the bending properties of the reinforcing structure are improved by the elastic and plastic elongation provided by the elongation zones below F2. However, when the stiff composite article is subjected to an impact force, e.g. a force between F2 and F3, the stiff composite article will be able to absorb the impact energy in a reversible change of its shape. This because mainly the reinforcing structure will stretched under an essentially elastic elongation.
Such forming processes, in which the reinforcing structure and the polymer matrix are subjected to heat and/or pressure in order to form a stiff composite article, may comprise one or subsequent steps.
First, polymer material, to be used as polymer matrix in the stiff composite article, is added to the reinforcing structure. E.g. several essentially parallel metallic elements are laminated between two or more polymer layers. Eventually, other elements of the reinforcing structure are added before or during laminating. An alternative is to extrude polymer material around the reinforcing structure, comprising the metallic elements, which are essentially parallel to each other. This combination of polymer material and reinforcing structure comprising metallic elements is subjected to a forming process.
Such a forming process could be e.g. press-forming, flow moulding, thermofolding or membrane forming.
In a press-forming operation (stamping), a reinforcing structure and the polymer matrix are heated to processing temperature and stamped to shape in a two-part-tool under pressure to provide a stiff composite article.
In a flow moulding operation, the reinforcing structure and the polymer matrix are placed in a two-part-tool and heated to the melting temperature of the polymer material. Eventually pressure is used. The mould cavity is filled by polymer matrix, flowing into every recess of the mould, so providing a stiff composite article.
In a thermofolding operation the reinforcing structure and the polymer matrix are heated locally and folded to provide a stiff composite article. In a membrane forming operation an autoclave pressure is used to drape the preheated reinforcing structure and the polymer matrix over a tool, providing a stiff composite article.
Further, the presence of metallic elements in the matrix during bending operations improves the heat distribution throughout the whole matrix. Since the metallic elements have a high thermal and electric conductivity. This leads to shorter cycle times compared to composite materials with other reinforcing structures, not comprising metallic elements such as glass fibers or carbon fibers.
It is also obvious that, when metallic elements are used in only one direction of the reinforcing structure, the structural deformations of the metallic elements will lead to an improvement of the bending properties of the reinforcing structure only in the direction parallel to the axis of the metallic element.
In case metallic elements are used in more than one direction each direction present in a different reinforcing layer of the reinforcing structure, the structural deformation of the metallic elements will lead to an improvement of the bending properties in all directions parallel to an axis of a metallic element.
The metallic elements can be woven or braided together with polymer filaments, yarns or polymer tapes into one and the same fabric. The metallic elements run in only one direction of the fabric, either the warp or weft direction. This fabric itself forms part of the reinforcing structure. It provides an alternative way to introduce the metallic elements into the polymer matrix. Polymer material, to be used as the polymer matrix of the stiff composite material itself, is added, e.g. extruded on one or on both sides of the fabric. Alternatively, the fabric may be laminated with one or more layers of polymer material. When such a combination of a fabric comprising metallic elements and polymer material is subjected to a forming process, in order to obtain a stiff composite article, the polymer filaments or yarns help to fill the spaces between the adjacent metallic elements. Especially when the filaments or yams are of the same polymer nature as compared to the polymer matrix.
Another alternative is provided by providing a polymer coating around the metallic elements, preferably before they are given the structural deformation. E.g. polymer material is extruded around the metallic element. Such polymer coating helps to improve the adhesion between the metallic elements of the reinforcing structure and the polymer matrix.
Any stiff thermoplastic material can be used as a polymer matrix. A thermoplastic material has to be considered as any thermoplast and any thermoplastic elastomer.
Examples of suitable thermoplastic materials are: polyethylene (PE), polypropylene (PP), polystyrene (PS), polyethylene terephthalate (PET), polyethylene napthalate (PEN), polybuteen terephthalate (PBT) polyvinylchloride (PVC), polyamide (PA), polyester (PES), polyimide (PI), polycarbonate (PC), styrene acrilonitryl (SAN), acrylonitril-butadiene-styrene (ABS), thermoplastic polyurethane (TPU), thermoplastic polyolefins (TPO), thermoplastic copolyetheresters, copolymers of these polymers or similar materials.
These polymer matrices could be defined by a Young""s modulus of more than 107 Pa, preferably between 108 Pa and 5*109 Pa. The Young""s modulus is measured at the origin of the stress-strain curve of the polymer at ambient temperatures.
A similar stiff composite article can be obtained by using thermoset polymers by forming the reinforcing structure without polymer matrix, and adding the thermoset matrix, being the polymer matrix, to the formed reinforcing structure by e.g. injection molding.
In case metallic elements, being strands or cords, are used in the reinforcing structure, either as such or integrated into a woven or braided fabric together with polymer filaments, yarns or tapes, preference is to be given to those strands or cords, which have a large and rough surface so as to increase the mechanical anchoring in the matrix. Examples are 3xc3x973 and 7xc3x973 cords, preferred to single stranded cords.
To improve the corrosion resistance of the metallic elements, the metallic elements can be coated with a metallic coating layer such as zinc or a zinc alloy such as brass. A suitable zinc alloy is an alloy comprising 2 to 10% Al and 0.1 to 0.4% of a rare earth element such as La and/or Ce.
In order to assure a good adhesion between the metallic and the thermoplastic material, an adhesion promoter can be applied on the metallic element.
Possible adhesion promoters are bifunctional coupling agents such as silane compounds. One functional group of these coupling agents is responsible for the binding with the metal or metal oxides; the other functional group reacts with the polymer.
More details about these coupling agents can be found in the PCT application WO-A-99/20682.
Other suitable adhesion promoters are aluminates, zirconates or titanates.
Wires used as metallic elements as such or comprised in a strand or cord may have various cross-sections and geometries, e.g. circular, oval or flat.
Within the range of wires, strands and cords, a large variety of materials can be used, dependent on the required mechanical strength. Wires used as such or comprised in a strand or cord, can have a diameter of 0.04 mm to 1.0 mm, preferably between 0.1 mm and 0.4 mm, e.g. 0.15 mm, 0.175 mm or 0.3 mm.
Wires having structural deformations, may also be used to provide a strand or cord. These strands or cords may form part of the reinforcing structure.
Any metal can be used to provide the metallic elements. Preferably alloys such as high carbon steel alloys or stainless steel alloys are used.
When steel wire is used, the tensile strength of the steel wires ranges from 1500 N/mm2 to 3000 N/mm2 and even more, and is mainly dependent upon the composition of the steel and the diameter.
Also other parameters of a strand or cord, such as the construction of the strand or cord, the number of wires and the diameters of each wire comprised in a strand or cord, the force at rupture of each wire comprised in the strand or cord, can be chosen to provide the required mechanical properties such as strength and elongation at rupture.